Recent imaging data of adolescent male and female brains also indicate gender differences in anatomical development. Gray matter volumes tend to peak in nearly all cortical and subcortical brain regions for girls an average of 1–3 years earlier than boys (Lenroot et al., 2007). For example, gray matter volume in the frontal lobes peaks at age 9.5 for girls and 10.5 for boys (Geidd, 2008). As discussed, executive functioning abilities are believed to rely on the development of frontal lobe circuitry. Moreover, the basal ganglia, a group of subcortical nuclei comprised of the caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra, are also involved in higher cognitive functions as well as the mediation of affective states and attention. Preliminary MRI data parallel the cortical gender differences, with gray matter in the caudate peaking at 10.5 years in girls and 14.0 years in boys (Geidd, 2008). Geidd (2008) purports that these differences in the trajectories of adolescent brain development for boys and girls has immediate utility for establishing endophenotypes, biomarkers used to divide behavioral symptoms into genetically based phenotypes. Since
